The invention relates to a system for a focused multi-planar image acquisition in a prober, which is provided with a movement device and a clamping fixture, which is mounted on said movement device and is intended for a test object. Furthermore, the prober exhibits probe needles, which can make contact with the test object; holding devices for the probe needles; and a clamping plate, which is arranged above the clamping fixture and on which the holding devices can be mounted and which exhibits a viewing aperture, which visibly exposes the surface of the test object. The system is provided with an image capturing device, which is mounted above the viewing aperture and which comprises a microscope with an objective lens and an objective holder. Said system is also provided with a lighting device, which can generate a light beam, directed on the surface of the test object.
The invention also relates to a method for a focused multi-planar image acquisition in a prober. According to the method, when the surface of a test object is positioned laterally in relation to the tips of the separated probe needles, a microscope is focused on the surface of the test object at a first time and on the plane of the probe needles at a second time.
There exist so-called confocal microscopes in order to generate three-dimensional images of microscopically small objects, like cells, pollen grains or the like. A confocal microscope is a variant of the light microscope, with which virtual optical cross sectional scans of an object can be produced. These cross sectional scans are then assembled with suitable software into a three dimensional display.
In these confocal microscopes the light falls through an aperture diaphragm on the specimen via a beam splitter mirror and an objective lens. The emitted light is captured by the objective lens and focused on the aperture of an aperture diaphragm, behind which is located the detector. The aperture diaphragm causes the beams from layers, which are located higher or lower than the focal plane, for which the objective lens is set, to be blanked out. The result is an image that matches a sectioning of the specimen in the focal plane. In order to obtain a three dimensional image with confocal microscopes, the image of this focal plane is stored. Then the focus of the objective lens is put into another focal plane, which is at a distance from the first focal plane. Thereupon an image, which matches a cross sectional scan of the specimen in this other focal plane, is generated and stored. After several repetitions of this procedure, a three dimensional image of the specimen is generated from the individual images from the various focal planes with suitable software.
In order to adjust the focus of the objective lens of the confocal microscope it is known to use microscope objective lens focusing systems as fast and compact adjustment units, which can be easily installed in most microscopes. In so doing, they are screwed in between the turret and the objective lens. To this end, such a microscope objective lens focusing system exhibits a turret-sided threaded shaft, which can be screwed into the objective thread of the microscope turret. Then the objective lens itself can be screwed into an internal thread in the microscope objective lens focusing system.
The threaded shaft and the internal thread can be moved in the axial direction in relation to each other by means of a position-encoded piezoelectric linear actuator. By applying various voltages to the piezoelectric linear actuator, the distance between the objective lens and the turret head and, thus, the focus can be modified by electric means. Hence, a suitable focal plane can be selected while scanning with a confocal microscope by varying the voltage in the microscope objective lens focusing system.
In these confocal microscopes, the object to be examined is illuminated with a laser beam. In this case the laser beam is expanded in such a manner that it illuminates the entire object. Another possibility with respect to a total illumination of the object is not to fan out, but rather to deflect temporally the laser beam along the line of a scanner mode and, thus, to illuminate the object by means of a scanned laser beam.
There also exist white light confocal microscopes, with which a color image is also possible, but with a lower intensity.
There exist so-called probers to test components, in particular semiconductor components, for their operational function and the impact that physical parameters have on them. A prober generally consists of a movement device, for example, an X-Y cross-table, which can also perform slight rotary motions in order to correct the position. This movement device has a clamping fixture—a so-called chuck. Hence, the test object can be mounted on this clamping fixture. Above the clamping fixture is a clamping plate, which is provided with a feed-through and viewing aperture. At this stage holding devices for the probe needles can be mounted on this clamping plate, for example, by means of vacuum holders, so that the probe needles can extend through the feed-through and viewing aperture and make an electric contact at the corresponding points on the object to be tested, thus measuring the object to be tested for its electrical properties.
There also exist solutions with so-called probe cards, where the probe needles are mounted, as the probe card needles, securely on the probe card; and then the probe card is placed securely in the clamping plate.
Basically the objects to be tested have to be positioned in relation to the tips of the probe needles. To this end, a vertical motion of the movement device usually brings about a separation between the object to be tested and the probe needles. The distance between the tips of the probe needles and the test object ranges usually from 200 to 250 μm. Then the movement device moves the clamping fixture and, thus, the object to be tested in such a manner that another test object or another part of the test object comes to rest under the tips of the probe needles. Thereafter, an additional vertical motion places again the object to be tested in contact with the tips of the probe needles. As a rule, this positioning operation is controlled by an image capturing device, which comprises a microscope and usually also a video camera. The image capturing device—that is, the objective lens of the microscope—is mounted above the viewing aperture. The image capturing device photographs the surface of the object to be tested, which in turn is passed to an image evaluating unit. Then with the image evaluating unit it is possible to control with suitable analysis programs the movement device so accurately that the tips of the probe needles come to rest directly over the corresponding contacts on the test object; and the measuring process can begin.
In the case of manual probers, the viewing takes place either by way of a monitor, which reproduces and enlarges the video camera-generated image, or directly at the eyepiece of the microscope by an operator, who then controls the positioning and the electrical contacting in accordance with the indicated position.
For a sharp setting of the image captured by the microscope, this microscope is provided with a vertical adjustment drive, by means of which an adjustment of the entire microscope changes the distance between the objective lens and the imaging plane.
Before the tips of the probe needles make contact with the contacts on the test object, the tips and the contact surface lie in different imaging planes with respect to the objective lens. As a result, a sharp image of both the tips and the contact surface can never be obtained simultaneously.
In the case of a manual prober the operator's first step is to guide the contact pads of the test object under the probe needles, which can be seen only as very ill-defined shadows. Then the second step is to decrease the distance between the probe needles and the test object. This action will bring the probe needles sharper into focus, if the microscope is focused on the test object. Hence, it is still feasible to make some subsequent adjustments. However, it is not until contact has actually been made that it is possible to determine exactly whether the tips of the contact needles are, in fact, exactly on the contact pads of the test object. If necessary, the operator has to break the contact once again, re-position, and then make contact again.
In the case of probers, where the microscope image is mapped in an image evaluating device, both objects can be brought into sharp focus by capturing the image of the wafer surface in a first step. Then in a second step after mechanical adjustment of the microscope the sharp image of the probe tip is captured. Thereupon the two images are stacked one on top of the other and assembled into one sharp composite image. Thus, a sharp image of both the probe needles and the test object is produced, however, this applies only to the static image, thus not to the real time mode.